<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .parent{

            display: flex;

            /* flex-grow:0  flex-shrink:0 flex-basis:auto*/
           /* flex:none;*/

            /*1，1，auto*/
            flex:1;
            width:550px;
            height:380px;
            background-color: darkblue;
        }
        .item{

            width:100px;
            height:100px;
            background-color: darkgoldenrod;
        }
        .parent .item:nth-child(3){
        flex:2;/*多分一份剩余空间*/
        background-color: #004b00;

        }

    </style>
</head>
<body>
<div class="parent">
    <div class="item">1</div>
    <div class="item" style="background-color: saddlebrown">2</div>
    <div class="item">3</div>
    <div class="item">1</div>
    <div class="item" style="background-color: saddlebrown">2</div>

</div>
</body>
</html>